Diagnostic assays are widespread and central for the diagnosis, treatment and management of many diseases. Different types of diagnostic assays have been developed over the years in order to simplify the detection of various analytes in clinical samples such as blood, serum, plasma, urine, saliva, tissue biopsies, stool, sputum, skin or throat swabs and tissue samples or processed tissue samples. These assays are frequently expected to give a fast and reliable result, while being easy to use and inexpensive to manufacture. Understandably it is difficult to meet all these requirements in one and the same assay. In practice, many assays are limited by their speed. Another important parameter is sensitivity. Recent developments in assay technology have led to increasingly more sensitive tests that allow detection of an analyte in trace quantities as well the detection of disease indicators in a sample at the earliest time possible.
A common type of disposable assay device includes a zone or area for receiving the liquid sample, a conjugate zone also known as a reagent zone, and a reaction zone also known as a detection zone. These assay devices are commonly known as lateral flow test strips. They employ a porous material, e.g., nitrocellulose, defining a path for fluid flow capable of supporting capillary flow. Examples include those shown in U.S. Pat. Nos. 5,559,041, 5,714,389, 5,120,643, and 6,228,660 all of which are incorporated herein by reference in their entireties.
The sample-addition zone frequently consists of a more porous material, capable of absorbing the sample, and, when separation of blood cells is desired, also effective to trap the red blood cells. Examples of such materials are fibrous materials, such as paper, fleece, gel or tissue, comprising e.g. cellulose, wool, glass fiber, asbestos, synthetic fibers, polymers, or mixtures of the same.
Another type of assay device is a non-porous assay device having projections to induce capillary flow. Examples of such assay devices include the open lateral flow device as disclosed in PCT International Publication Nos. WO 2003/103835, WO 2005/089082, WO 2005/118139, and WO 2006/137785, all of which are incorporated herein by reference in their entireties.
A known non-porous assay device is shown in FIG. 1. The assay device 1, has at least one sample addition zone 2, a reagent zone 3, at least one detection zone 4, and at least one wicking zone 5. The zones form a flow path by which sample flows from the sample addition zone to the wicking zone. Also included are capture elements, such as antibodies, in the detection zone 4, capable of binding to the analyte, optionally deposited on the device (such as by coating); and a labeled conjugate material also capable of participating in reactions that will enable determination of the concentration of the analyte, deposited on the device in the reagent zone, wherein the labeled conjugate material carries a label for detection in the detection zone. The conjugate material is dissolved as the sample flows through the reagent zone forming a conjugate plume of dissolved labeled conjugate material and sample that flows downstream to the detection zone. As the conjugate plume flows into the detection zone, the conjugated material will be captured by the capture elements such as via a complex of conjugated material and analyte (as in a “sandwich” assay) or directly (as in a “competitive” assay). Unbound dissolved conjugate material will be swept past the detection zone into the at least one wicking zone 5. Also shown in FIG. 1 are projections or micropillars 7.
An instrument such as that disclosed in US Patent Publication Nos. US20060289787A1 and US 20070231883A1, and U.S. Pat. Nos. 7,416,700 and 6,139,800, all of which are incorporated herein by reference in their entireties, is able to detect the bound conjugated material in the detection zone. Common labels include fluorescent dyes that can be detected by instruments which excite the fluorescent dyes and incorporate a detector capable of detecting the fluorescent dyes.
The sample size for such typical assay devices as shown in FIG. 1 are generally on the order of 200 μl. Such a sample size requires a venous blood draw from a medical professional such as a phlebotomist. There is an increasing need for lateral flow devices that are able to function with a much smaller sample size to accommodate the amount of blood available from a so-called “fingerstick” blood draw, which is on the order of 25 μl or less. Such a small amount of sample is the amount of blood in a drop of blood after pricking a finger tip with a lancet. Home blood glucose meters typically use a drop of blood obtained in such a fashion to provide glucose levels in blood. Such a smaller sample size would not require a medical professional to draw the blood and would provide greater comfort to the patients providing the sample for analysis.
To reduce sample size required, the dimensions of the lateral flow assay devices are reduced to accommodate the smaller sample size. However, it has been found that reducing the sample size and dimensions of the device provides inadequate conjugate in the detection zone and accordingly less signal that can be read by the instrument (in some instances up to a 5×lower signal) and poor sensitivity. The inadequate conjugate in the detection zone is believed to be due to reduced sample size and inefficient use of the sample in the device, amongst other conditions. Another drawback of reducing dimensions is that the width of the detection zone will also be reduced, again making less signal available that can be read by the instrument. Also, it has been found that a smaller device has reduced flow time and conjugate material contact time, resulting in less binding between the analyte in the sample and the conjugate material.
A need continues to exist for smaller sample volume assay devices that can accommodate smaller and smaller sample sizes, can accommodate various samples (such as whole blood), and can provide results with the required sensitivity and specificity.